This invention relates to optical filters and more particularly to acoustically tuned optical filters.
Optical filters that pass or reject a band of wavelengths around a preselected central wavelength are used in some modern optical systems. These prior art optical filters allow this central wavelength, and pertinent characteristics of the filter, to be changed electrically.
In a typical prior art optical filter of the type referred to hereinabove, a single crystal having birefringent characteristics is used and is electrically excited by suitable means. The excited crystal, when illuminated by light which is polarized parallel to the crystal's extraordinary axis, shifts a fraction of the light energy (in a narrow wavelength band) to the ordinary axis. Light outside of that wavelength band is unaffected. Such a typical prior art optical filter can shift perhaps 70 percent or more of the light energy, but a 100 percent shift has not been attainable, although desirable (and in fact necessary) in some situations. Stated another way, a 90 degree rotation of the plane of polarization cannot be made. As a result, the in-band and the out-of-band light have polarization planes less than 90 degrees apart, so that the rejection or the transmission is incomplete with these prior art optical filters.